incorrect arunforce, PNG is widely beleived to be the best quality. i also beleive this

BMP- bitmap, has a bad quality compared to the rest. it is a low res and low file sized option. and they have a bad icon. avoid in general

JPG- also known as JPEG, i cant remember the full name but it does have a medium quality. probably the widest accepted format for sigs and the like, you wont go wrong with a JPG but its not the best and i dont like JPG format.

PNG- pixar node graphic, it is by far the best in my opinion. it has interlacing options for faster loads, most sigs onl get up to 156kb using PNG format AND they have a liquid smooth presentation. i use them becasue i use detail, and PNG shows detail the best.

GIF- bever use a GIF unless its an animation. GIF stands for Graphic Interchange Format, and means animated basically. It lowers the colour range to allow room for animation, so if you have two slightly different oranges side by side they will merge to the closest orange and therefore you lose alot of quality. pixellation is also common